When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating rays).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating ray to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has a basic structure comprising a support and a stimulable phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective film is normally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor dispersed therein. However, there is a known phosphor layer comprising agglomerate of a stimulable phosphor without binder. The phosphor layer containing no binder can be formed by a vapor phase deposition method or by a firing method. It is also known to impregnate with a polymer material the phosphor layer comprising stimulable phosphor agglomerate.
JP 2001-255610 A discloses the radiation image recording and reproducing method of another type. While a stimulable phosphor of the storage panel used in the conventional type has both of radiation-absorbing function and energy-storing function, those two functions are separated in the disclosed method. In the method, a radiation image storage panel comprising a stimulable phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of causing the radiation-absorbing phosphor of the screen to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storing phosphor (stimulable phosphor) of the storage panel to store the energy of the converted light as radiation image information; sequentially exciting the stimulable phosphor with a stimulating light to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric signals giving a visible radiation image. The present invention can be also applied to the radiation image storage panel used in this type of the method.
In any case, it is desired that radiation image storage panels have sensitivity as high as possible and further can give a reproduced radiation image of high quality (in regard of sharpness and graininess).
In order to improve the sensitivity and the image quality, it has been proposed that the phosphor layer of the storage panel be prepared by a vapor phase deposition method such as vacuum vapor deposition or sputtering. The process of vacuum vapor deposition, for example, comprises the steps of heating an evaporation source comprising the phosphor or materials thereof for vaporization by means of a resistance heater or an electron beam, and depositing the vapor on a substrate such as a metal sheet to form a film (layer) of the phosphor in the form of columnar crystals.
The phosphor layer formed by the vapor phase deposition method contains no binder and consists of the phosphor only, and there are cracks among the columnar crystals of the phosphor. Because of the presence of the cracks, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too. Accordingly, a radiation image storage panel having the phosphor layer formed by the vapor phase deposition method has high sensitivity. At the same time, since the cracks prevent the stimulating light from diffusing parallel to the phosphor layer, the storage panel can give a reproduced image with a high sharpness.
WO 01/03156A1 discloses a CsX:Eu stimulable phosphor and a phosphor screen having a phosphor layer thereof formed by a vapor phase deposition method. The publication also discloses a ultraviolet light(280 nm)-excited instant emission spectrum of a phosphor film comprising a CsBr:Eu stimulable phosphor formed by the vapor deposition method. In the spectrum, there is a sharp peak at approx. 440 nm but no emission peak is observed in the wavelength region of 400 to 420 nm.